Hey,
Here is the user guide for aspire 5520
You will need to open a new internet page, and copy this text fully into the addressbar to download the guide, starting with ftp:
ftp://ftp.work.acer-euro.com/notebook/aspire_5520/manual/Aspire_5220_5520_5520G_EN.zip

The zip file has the pdf format manual inside, if you can't open it let me know.

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Remove the three black screws under the battery, not the silver screws.

VERY carefuly pry off the power button cover. It will make 'snapping' sounds and if you where gentle nothing will be broken.

The keyboard has no screws holding it and can be slid toward the screen and very gently pryed out, taking care not to tear the ribbon cable, as mentioned before.

When the keyboard is out the cable can be released by gently pulling the cable lock on the header toward the cable, the thin (black in this case, it's usualy a different color than the white connector) plastic part of the connector I just use my fingernails, one index finger on each end slowly pulling till it releases.
Pull the cable out using the pull tab. You can just pull the flat cable out with the pull tab without releasing the calbe lock but you stil need to release the cable lock to properly put in the new cable, then push the cable lock back tight to the connector.

you should have a button on your keyboard probably in blue called Fn. once you press this you need to press the screen tab button on the keyboard that corresponds to the screen. This is usually present on one of the function buttons from F1 to F9.

1)Try first in safe mode (F8 key) and try to change password from there.
2) how many user account you can see on ur login page. If its more then one then you can go in other account and change your password from there .
3) If you have windows product key sticker attached on side then just borrow any windows XP (but same OS you have in ur computer) from your friend. Run that disk and reinstall your windows with your product key.
Hope it helps !!
Please let me know so that we can try some more tips ..